Fine pitch microcontacts and method for forming thereof

ABSTRACT

A method includes applying a final etch-resistant material to an in-process substrate so that the final etch-resistant material at least partially covers first microcontact portions integral with the substrate and projecting upwardly from a surface of the substrate, and etching the surface of the substrate so as to leave second microcontact portions below the first microcontact portions and integral therewith, the final etch-resistant material at least partially protecting the first microcontact portions from etching during the further etching step. A microelectronic unit includes a substrate, and a plurality of microcontacts projecting in a vertical direction from the substrate, each microcontact including a base region adjacent the substrate and a tip region remote from the substrate, each microcontact having a horizontal dimension which is a first function of vertical location in the base region and which is a second function of vertical location in the tip region.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 11/717,587, filed Mar. 13, 2007, which application is acontinuation-in-part of U.S. patent application Ser. No. 11/166,982,filed Jun. 24, 2005, which application claims the benefit of the filingdate of U.S. Provisional Patent Application No. 60/583,109, filed Jun.25, 2004. Application Ser. No. 11/166,982 is also a continuation-in-partof U.S. patent application Ser. No. 10/959,465, filed Oct. 6, 2004.Application Ser. No. 10/959,465 also claims the benefit of the filingdates of U.S. Provisional Patent Application Nos. 60/508,970, filed Oct.6, 2003; 60/533,210, filed Dec. 30, 2003; 60/533,393, filed Dec. 30,2003; and 60/533,437, filed Dec. 30, 2003. The disclosures of all of theaforementioned applications are hereby incorporated by reference herein.

BACKGROUND OF THE INVENTION

The present invention relates to microelectronic packages, to componentsfor use in fabrication of microelectronic packages, and to methods ofmaking the packages and component.

Microcontact elements in the form of elongated posts or pins may be usedto connect microelectronic packages to circuit boards and for otherconnections in microelectronic packaging. In some instances,microcontacts have been formed by etching a metallic structure includingone or more metallic layers to form the microcontacts. The etchingprocess limits the size of the microcontacts. Conventional etchingprocesses typically cannot form microcontacts with a large ratio ofheight to maximum width, referred to herein as “aspect ratio”. It hasbeen difficult or impossible to form arrays of microcontacts withappreciable height and very small pitch or spacing between adjacentmicrocontacts. Moreover, the configurations of the microcontacts formedby conventional etching processes are limited.

For these and other reasons, further improvement would be desirable.

SUMMARY OF THE INVENTION

In one embodiment, a method of forming microcontacts, includes, (a)providing a first etch-resistant material at selected locations on a topsurface of a substrate, (b) etching a top surface of the substrate atlocations not covered by the first etch-resistant material and therebyform first microcontact portions projecting upwardly from the substrateat the selected locations, (c) providing a second etch-resistantmaterial on the first microcontact portions, and (d) further etching thesubstrate to form second microcontact portions below the firstmicrocontact portions, the second etch-resistant material at leastpartially protecting the first microcontact portions from etching duringthe further etching step.

In another embodiment, a method of forming microcontacts, includes (a)applying a final etch-resistant material to an in-process substrate sothat the final etch-resistant material at least partially covers firstmicrocontact portions integral with the substrate and projectingupwardly from a surface of the substrate, and (b)

etching the surface of the substrate so as to leave second microcontactportions below the first microcontact portions and integral therewith,the final etch-resistant material at least partially protecting thefirst microcontact portions from etching during the further etchingstep.

In still another embodiment, a microelectronic unit includes (a) asubstrate, and (b) a plurality of microcontacts projecting in a verticaldirection from the substrate, each microcontact including a base regionadjacent the substrate and a tip region, remote from the substrate, eachmicrocontact having a horizontal dimension which is a first function ofvertical location in the base region and which is a second function ofvertical location in the tip region.

In yet another embodiment, a microelectronic unit includes a substrate,a plurality of microcontacts projecting in a vertical direction from thesubstrate wherein a pitch between two adjacent microcontacts is lessthan 150 microns.

In still another embodiment, a microelectronic unit includes (a) asubstrate, and (b) a plurality of elongated microcontacts projecting ina vertical direction from the substrate, each microcontact including abase region adjacent the substrate and a tip region, remote from thesubstrate, each microcontact having an axis and a circumferentialsurface which slopes toward or away from the axis in the verticaldirection along the axis, such that the slope of the circumferentialwall changes abruptly at a boundary between the tip region and the baseregion.

In another embodiment, a microelectronic unit includes (a) a substrate,and (b) a plurality of microcontacts projecting in a vertical directionfrom the substrate, each microcontact having a proximal portion adjacentthe substrate and an elongated distal portion extending from theproximal portion in the vertical direction away from the substrate, thewidth of the post increasing in stepwise fashion at the juncture betweenthe proximal and distal portions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a substrate.

FIG. 2 is a schematic illustration of the substrate of FIG. 1 with alayer of photoresist.

FIG. 3 is a perspective schematic illustration of the substrate of FIG.1 with a layer of photoresist and a mask.

FIG. 4 is a schematic illustration of the substrate of FIG. 1 beingetched.

FIG. 5 is a schematic illustration of the substrate of FIG. 1 with asecond photoresist.

FIG. 6 is a schematic illustration of the substrate of FIG. 1 having thesecond photoresist developed.

FIG. 7 is a schematic illustration of the substrate of FIG. 1 beingetched a second time.

FIGS. 8A-8D are example profiles of microcontacts.

FIG. 9 is a flowchart depicting a first embodiment.

FIG. 10 is a flowchart depicting a second embodiment.

FIG. 11 is a schematic illustration of a multi-layer substrate inapplication.

FIG. 12 is a schematic illustration of microelectronic unit.

FIG. 13 is a schematic illustration of two adjacent microelectronicunits.

FIG. 14 is a schematic illustration of a microelectronic assembly.

FIG. 15 is another schematic illustration of a microelectronic assembly.

FIG. 16 is yet another schematic illustration of a microelectronicassembly.

DETAILED DESCRIPTION

A first method or embodiment is described. FIG. 1 is a schematicillustration of a tri-metal substrate 10. The tri-metal substrate 10 hasa trace layer 12, an etch stop layer 14, a thick layer 16, and a topsurface 18. The trace layer 12 and the thick layer 16 may be formed of areadily etchable first metal such as copper, while the etch stop layermay be formed of a metal, such as nickel, which is substantiallyresistant to etching by a process used to etch copper. Although, copperand nickel are recited, the substrate 10 may be formed of any suitablematerial as desired.

FIG. 2 is a schematic illustration of the tri-metal substrate 10 of FIG.1 with a layer of a first photoresist 20. The first photoresist 20 isdeposited onto the top surface 18. The first photoresist 20 may be anytype of material that hardens or undergoes a chemical reaction whenexposed to radiation such as light. Thus, any etch-resistant materialmaybe used. Positive and negative photoresists may also be utilized andare known in the art. As used herein, the terms “top,” “bottom” andother directional terms are to be taken as relative to themicroelectronic element, rather than directions based on gravity.

FIG. 3 is a perspective schematic illustration of the tri-metalsubstrate of FIG. 1 with the layer of first photoresist 20 and a mask22. The mask 22 is often a transparent plate with opaque areas printedon it called a photomask or shadowmask, creating a pattern 24 on themask 22 with areas covered by the mask 22, denoted by reference numeral26, and areas not covered by the mask 22, denoted by reference numeral28. The pattern 24 with the covered and uncovered areas, 26 and 28,respectively, allows for selectively exposing parts of the firstphotoresist 20 to radiation.

Once the mask 22 is placed atop the first photoresist 20, radiation isprovided. Most often the radiation is in the form of ultraviolet light.This radiation exposes the first photoresist 20 at the uncovered areas28 resulting in making the uncovered areas 28 insoluble. The opposite istrue when a negative photoresist is used: the covered areas 26 becomeinsoluble. After exposing the first photoresist 20, the mask 22 isremoved. The first photoresist 20 is then developed by washing with asolution which removes the first photoresist 20 in the locations wherethe first photoresist 20 has not become insoluble. Thus, the photoresistexposure and development leaves a pattern of insoluble material on thetop of surface 18 of the substrate 10. This pattern of insolublematerial mirrors the pattern 24 of the mask 22.

After exposure and development of the photoresist, the substrate isetched as shown in FIG. 4. Once a certain depth of etching is reached,the etching process is interrupted. For example, the etching process canbe terminated after a predetermined time. The etching process leavesfirst microcontact portions 32 projecting upwardly from substrate 10 atthe thick layer 16. As the etchant attacks the thick layer 16, itremoves material beneath the edges of first photoresist 20 allowing thefirst photoresist 20 to project laterally from the top of firstmicrocontact portions 32, denoted as overhang 30. The first photoresist20 remains at particular locations as determined by the mask 22.

Once the thick layer 16 has been etched to a desired depth, a secondlayer of photoresist 34 (FIG. 5) is deposited on the tri-metal substrate10. In this instance, the second photoresist 34 is deposited onto thethick layer 16 at the locations where the thick layer 16 has beenpreviously etched. Thus, the second photoresist 34 also covers the firstmicrocontact portions 32. If using electrophoretic photoresists, thesecond photoresist 34, due to its inherent chemical properties, does notdeposit onto the first photoresist 20.

At the next step, the substrate with the first and second photoresists,20 and 34 is exposed to radiation and then the second photoresist isdeveloped. As shown in FIG. 6, the first photoresist 20 projectslaterally over portions of the thick layer 16, denoted by overhang 30.This overhang 30 prevents the second photoresist 34 from being exposedto radiation and thus prevents it from being developed and removed,causing portions of the second photoresist 34 to adhere to the firstmicrocontact portions 32. Thus, the first photoresist 20 acts as a maskto the second photoresist 34. The second photoresist 34 is developed bywashing so as to remove the radiation exposed second photoresist 34.This leaves the unexposed portions of second photoresist 34 on the firstmicrocontact portions 32.

Once portions of the second photoresist 34 have been exposed anddeveloped, a second etching process is performed, removing additionalportions of the thick layer 16 of the tri-metal substrate 10, therebyforming second microcontact portions 36 below the first microcontactportions 32 as shown in FIG. 7. During this step, the second photoresist34, still adhered to first microcontact portions 32, protects the firstmicrocontact portions 32 from being etched again.

These steps may be repeated as many times as desired to create thepreferred aspect ratio and pitch forming third, fourth or nthmicrocontact portions. The process may be stopped when the etch-stoplayer 14 is reached. As a final step, the first and second photoresists20 and 34, respectively, may be stripped entirely.

These processes result in microcontacts 38 shown in FIGS. 8A through 8D.These figures also illustrate the various profiles that may be achievedusing the processes described herein. Referring to FIGS. 8A-8C, themicrocontacts 38 have a first portion 32, also known as a tip region,and a second portion 36, also referred to as the base region. Providedthat the spots of first photoresist used in the steps discussed aboveare circular, each microcontact will be generally in the form of a bodyof revolution about a central axis 51 (FIG. 8A) extending in a verticalor Z direction, upwardly from the remainder of the substrate andgenerally perpendicular to the plane of the etch stop layer 14. Thewidths or diameters X of the first and second portions vary withposition in the Z or height direction within each portion. Statedanother way, within the first portion, X=F₁(Z), and within the secondportion X=F₂(Z). The slope or

$\frac{X}{Z}$

may change abruptly at the boundary 52 between the first and secondportions. The particular functions and hence the shape of themicrocontacts are determined by the etching conditions used in the firstand second etching steps. For example, the composition of the etchantand etching temperature can be varied to vary the rate at which theetchant attacks the metal layer. Also, the mechanics of contacting theetchant with the metal layer can be varied. The etchant can be sprayedforcibly toward the substrate, or the substrate can be dipped into theetchant. The etching conditions may be the same or different duringetching of the first and second portions.

In the microcontacts shown in FIG. 8A, the first portion 32 has acircumferential surface 44 which flares outwardly in the downwarddirection, so that the magnitude of the slope or

$\frac{X}{Z}$

increases in the downward direction. The second portion 36 also has acircumferential surface 46 flares outwardly; the magnitude of the slopeor

$\frac{X}{Z}$

of the second is at a minimum at boundary 52, and progressivelyincreases in the direction toward the base of the post. There is asubstantially change in slope at boundary 52. The maximum width ordiameter X of the second portion, at the base of the microcontact wherethe microcontact joins layer 14, is substantial greater than the maximumwidth or diameter of the first portion. In FIG. 8B, the maximum width ofsecond portion 36 is only slightly greater than the maximum width offirst portion 32. Also, the second portion has a minimum width at alocation between the base of the post and the boundary 52, so that thewidth gradually decreases in the upward direction to the minimum andthen progressively increases in the upward direction from the minimum tothe boundary 52. Such a shape is commonly referred to as a “coolingtower” shape. In the microcontacts of FIG. 8B, the slope or

$\frac{X}{Z}$

changes sign at the boundary 52 between the portions. In FIG. 8C, thesecond portion 36 has its minimum width near the base of themicrocontact.

Lastly, FIG. 8D illustrates a profile of a microcontact 38 having morethan two portions. This type of profile may result in the event thesteps of the processes described herein are performed numerous times.Thus, it can be seen that this particular microcontact 38 has fourportions, the first and second portions 32 and 36, respectively, andthird and fourth portions, 40 and 42, respectively. These four portionsmay have any dimension and be wider or slimmer than another portion asdesired. In this instance, there may be greater than one boundary. FIGS.8A-8D are only representative profiles and a variety of profiles may beachieved.

Although arrays including only two microcontacts or posts are depictedin each of FIGS. 8A-8D, in practice, an array of posts includingnumerous posts can be formed. In the embodiments depicted in each ofFIGS. 8A-8D, all of the microcontacts or posts in the array are formedfrom a single metallic layer 16 (FIG. 1). Each microcontact overlies aportion of the etch stop layer 14 at the base of the microcontact, wherethe microcontact connects to metallic layer 12. As discussed below, theetch stop layer 14 typically is removed in regions between themicrocontact, and metallic layer 12 typically is etched or otherwisetreated to convert it into traces or other conductive features connectedto the microcontact. However, the body of each microcontact, from itsbase to its tip, is a unitary body, free of joints such as welds, andhaving substantially uniform composition throughout. Also, because thetip surfaces 18′ of the microcontacts, at the ends of the microcontactsremote from layers 12 and 14, are portions of the original top surface18 of metal layer 16 (FIG. 1), these tip surfaces are substantially flatand horizontal, and the tip surfaces of all of the microcontacts aresubstantially coplanar with one another.

In an alternate embodiment, rather than remove the first photoresist 20only at selected locations after the first etching step, the entirefirst photoresist 20 may be removed. In this instance, the secondphotoresist 34 may be deposited over the entire surface of the substrate10. Then the mask 22 is placed onto the second photoresist 34. The mask22 must be properly aligned so as to expose only at the locationspreviously exposed, on the first microcontact portions 32. The secondphotoresist 34 is then developed and further etching may be performed onthe substrate 10.

FIG. 9 is a flowchart depicting the first embodiment. Beginning at step100, a substrate is provided. Then at step 102, a photoresist n isdeposited onto the substrate. Then at step 104, a mask is placed atopthe photoresist n. At step 106 the photoresist n is exposed toradiation. Subsequently, at step 108 the mask is removed and then atstep 110, the photoresist n is developed at select locations and thesubstrate is etched.

Next, another photoresist is deposited, known as n+1 at step 112. Then,at step 114, this n+1 photoresist is exposed to radiation. Subsequently,at step 116, the photoresist n+1 is removed at select locations and thesubstrate is etched again. Then, it is evaluated whether the desiredmicrocontact height has been achieved at step 118. If the desiredmicrocontact height has not been achieved, at step 120, the processreturns to step 112 and another photoresist is deposited onto thesubstrate. If the desired height has been achieved at step 122, then theremaining photoresists are removed at step 124 and the process ends.

FIG. 10 is a flowchart depicting a second embodiment. Steps 200-210 ofthe second embodiment mirror steps 100-110 of the first embodiment.However, at step 212, the entire photoresist n is removed. Then, at step214, another layer of photoresist n+1 is deposited onto the substrate.Next, the mask is placed back onto the substrate at step 216. Duringthis step, the mask must be aligned such that its pattern is situated insubstantially the same location as when the mask was placed on thephotoresist n. Subsequently, at step 218, the photoresist n+1 is exposedto radiation and the mask is removed.

Next, at step 220, photoresist n+1 is selectively removed and thesubstrate is etched again. This process may also be repeated until thedesired microcontact height is achieved. Thus, at step 222, it isevaluated whether the desired microcontact height has been achieved. Ifthe preferred height has not been achieved at step 224, then the processreturns to step 212 where the photoresist is removed entirely andanother photoresist n+1 is deposited and the steps continue thereon.However, if the desired height has been achieved at step 224, theremaining photoresist is removed at step 228 and the process ends.

The etch-stop layer 14 and the thin layer 12 may be united with adielectric layer and then thin layer 12 may be etched to form traces soas to provide a component with the microcontacts connected to the tracesand with the microcontacts projecting from the dielectric layer. Such astructure can be used, for example, as an element of a semiconductorchip package. For example, U.S. patent application Ser. No. 11/318,822,filed Dec. 27, 2005, the disclosure of which is hereby incorporated byreference herein, may be used.

The structure described herein may be an integral part of a multilayersubstrate 10, for instance, the top layer of a multilayer substrate 10,as shown in FIG. 11. Microcontacts 38 may be soldered to the die 54. Thesolder 56 may wick around a portion of the microcontacts 38. Wickingprovides very good contact between the microcontacts 38 and the die 54.Other bonding processes besides solder 56 may also be used. Surroundingthe microcontacts 38 is underfill 58, used to adhere the die 54 to themicrocontacts 38 and the substrate 10. Any type of underfill 58 may beused as desired or underfill 58 may be omitted. Below the microcontacts38 are traces 60 and a dielectric layer 62. Terminals 64 are disposed atthe bottom of the substrate 10.

Certain packages include microelectronic chips that are stacked. Thisallows the package to occupy a surface area on a substrate that is lessthan the total surface area of the chips in the stack. Packages whichinclude microcontacts fabricated using the processes recited herein maybe stacked. Reference is made to co-pending U.S. patent application Ser.No. 11/140,312, filed May 27, 2005; and U.S. Pat. No. 6,782,610, thedisclosures of which are hereby incorporated by reference. Themicrocontact etching steps taught in these disclosures may be replacedby the processes discussed herein.

Although a tri-metal substrate is discussed above, a suitable substratehaving any number of layers may be utilized, such as for example asingle metal. Additionally, rather than use a photoresist, anetch-resistant metal such as gold or other metal substantially resistantto the etchant used to etch the thick metallic layer, may be used. Forexample, the etch-resistant metal can be used in place of the firstphotoresist 20 discussed above. Spots of etch-resistant metal may beplated onto the top of the thick layer 16 after applying a mask such asa photoresist with holes at the desired locations for the spots. Afterplating the etch-resistant metal onto the top of the thick layer, thethick layer is etched to form the microcontacts as discussed above. Theetch-resistant metal may be left in place on the tip of themicrocontact. In the event an etch-resistant metal is used, as a secondetch-resistant material (in place of second photoresist 34 discussedabove), a mask may be used to limit deposition of the secondetch-resistant metal to only the first portions 32 of the microcontacts,so that the areas between the microcontacts remain free of theetch-resistant metal. Alternately, the entire first layer ofetch-resistant metal may be removed upon etching first microcontactportions 32, then a second layer of etch-resistant metal may bedeposited to protect the first microcontact portions 32.

With reference to FIG. 12, a microelectronic unit 70 is shown havingmicrocontacts 72. The microcontacts 72 have an etch stop layer 74. Themicrocontacts 72 project vertically from a metallic layer that has beenformed into traces 76. There may be gaps or spaces 78 between the traces76. A first layer of dielectric 80 may be adhered to a bottom side ofthe unit 70 adjacent the traces 76. Openings 82 in the first layer ofdielectric 80 allow the traces 76 to form electronic contacts. A secondlayer of a dielectric 84 may be formed on a top side of the unit 70.

The microcontacts formed from these processes may have a typical heightranging from about 40 microns to about 200 microns. Further, the typicalpitch between microcontacts may be less than about 200 microns,preferably less than 150 microns. In particular, in reference to FIG.13, two microcontacts are shown having a tip diameter d and amicrocontact height h. A pitch P is defined by the distance between thelongitudinal axes of the two microcontacts.

In many applications, particularly where microcontacts are usedconnected to contacts of a semiconductor chip as, for example, in astructure as discussed below with reference to FIG. 14, it is desirableto provide a small pitch. However, in a process where the microcontactsare formed from a single metal layer by a single etching process, it isnormally not practical to make the pitch P less than a certain minimumpitch P₀ which is equal to the sum of the diameter d plus the height h.Thus, P₀=d+h. In theory, the minimum pitch could be reduced by reducingthe tip diameter d. However, it is impossible to make the tip diameterless than zero. Moreover, in many cases it is undesirable to reduce thetip diameter below about 20 or 30 microns. For example, the adhesionbetween the tips of the pins and spots of photoresist used to protectthe tips during etching is proportional to the area of the tips, andhence to the square of the tip diameter. Therefore, with very small tipdiameters, the photoresist spots can be dislodged during processing.Thus, using conventional processes, it has been difficult to formmicrocontacts with very small pitch.

However, the pitch between microcontacts using the process recitedherein can be less than P_(o), (P<P_(o)), for example, P=(0.9) P₀ orless. For instance, if the diameter d of the tip is 30 microns and theheight h is 60 microns, a conventional process would achieve a pitchP_(o) of 90 microns. However, the process described herein, with atleast two etches, can achieve a pitch P of about 80 microns or less.Stated another way, the multi-step etching process allows formation ofunitary metallic microcontacts or posts from a single metallic layerwith combinations of pitch, tip diameter and height not attainable inconventional etching processes. As the number of etching stepsincreases, the minimum attainable pitch for a given tip diameter andheight decreases.

Referring now to FIG. 14, a microelectronic package 90 is shown using apackage element or chip carrier having microcontacts 38 as discussedabove. The chip carrier includes a first dielectric layer 62 which maybe formed from a material such as polyimide, BT resin or otherdielectric material of the type commonly used for chip carriers. Thechip carrier also includes traces 60 connected to some or all of themicrocontacts 38. The traces incorporate terminals 61. The microcontacts38 project from a first side of dielectric layer 62, facing upwardly asseen in FIG. 14. Dielectric layer 62 has openings 82, and terminals 61are exposed at the second or downwardly facing surface of the firstdielectric layer 62 through openings 82. The carrier further includes anoptional second dielectric layer 84.

The tips of microcontacts 38 are bonded to contacts 55 of amicroelectronic element such as a semiconductor chip or die 54. Forexample, the tips of the microcontacts may be solder-bonded to thecontacts 55 of the microelectronic element. Other bonding processes,such as eutectic bonding or diffusion bonding, may be employed. Theresulting packaged microelectronic element has some or all of contacts55 on the microelectronic element connected to terminals 61 by themicrocontacts and traces. The packaged microelectronic element may bemounted to a circuit panel 92, such as a printed circuit board bybonding terminals 61 to pads 94 on the circuit board. For instance, pads94 on the circuit panel 92 may be soldered to the terminals 61, atopenings 82, using solder balls 96.

The connection between the microcontacts 38 and the contacts 55 of themicroelectronic element can provide a reliable connection even where thecontacts 55 are closely spaced. As discussed above, the microcontacts 38can be formed with reasonable tip diameters and height. The appreciabletip diameter can provide substantial bond area between the tip of eachmicrocontact and the contact of the microelectronic element. In service,differential thermal expansion and contraction of the chip 54 relativeto the circuit panel 92 can be accommodated by bending and tilting ofmicrocontacts 38. This action is enhanced by the height of themicrocontacts. Moreover, because the microcontacts are formed from acommon metal layer, the heights of the microcontacts are uniform towithin a very close tolerance. This facilitates engagement and formationof robust bonds between the microcontact tips with the contacts of thechip or other microelectronic element.

The structure of the chip carrier can be varied. For example, the chipcarrier may include only one dielectric layer. The traces may bedisposed on either side of the dielectric layer. Alternatively, the chipcarrier may include a multi-layer dielectric, and may include multiplelayers of traces, as well as other features such as electricallyconductive ground planes.

A process for further embodiment of the invention uses a structurehaving post portions 550 (FIG. 15) projecting from a surface 526 such asa surface of dielectric layer 510. Post portions 550 may be formed byany process, but desirably are formed by an etching process similar tothose discussed above. After formation of portions 550, a metallic orother conductive layer 502 is applied over the tips 533 of post portions550. For example, layer 502 may be laminated on the structureincorporating portions 550, and metallurgically bonded to the tips ofpost portions 550. Layer 502 is selectively treated so as to removematerial of the layer remote from post portions 550, but leave at leastpart of the layer thickness overlying post portions 550, and therebyform additional post portions 504 (FIG. 16) aligned with post portions550, and thus form composite microcontacts, each including a proximalpost portion 550 close to the substrate and a distal post portion 504remote from the substrate, the distal portion projecting in the verticalor z direction from the proximal portion. The treatment applied to layer502 may include an etching process as discussed above, using spots of anetch-resistant material 506 aligned with post portions 550. A protectivelayer such as a dielectric encapsulant 508 may be applied to cover postportions 550 before etching layer 502. Alternatively or additionally,post portions 550 may be plated or otherwise covered with anetch-resistant conductive material such as nickel or gold before etchinglayer 502.

The process of building up successive post portions may be repeated soas to form additional portions on portions 504, so that microcontacts ofessentially any length can be formed. The long microcontacts provideincreased flexibility and movement of the post tips. Where one or moredielectric encapsulant layers are left in place around thealready-formed post portions, such as layer 508 in FIGS. 15 and 16, theencapsulant desirably is compliant so that it does not substantiallylimit flexure of the posts. In other embodiments, the encapsulant isremoved before the components are used. Although the microcontacts areillustrated in conjunction with a dielectric substrate 522 and traces528 similar to those discussed above, this process can be used tofabricate microcontacts on essentially any structure.

As shown in FIG. 16, each microcontact has a horizontal or widthdimension x which varies over the vertical or z-direction extent of theproximal post portion 550 and which increases abruptly, in substantiallystepwise fashion, at the juncture between the proximal post portion 550and the distal portion 504, and varies along the vertical extent of thedistal portion. The slope of the variation in width with verticallocation also changes abruptly at the juncture between the postportions. The pattern of variation of the horizontal or width dimensionwithin each post portion depends upon the process used for etching orotherwise forming such post portion. For example, in a furtherembodiment, the distal post portions 504 may be formed by a multi-stageetching process as discussed above, so that each distal post portionincludes different sub-portions with different functions defining thevariation of width x in the vertical or z direction.

Reference is also made to the following, which are hereby incorporatedby reference: U.S. patent application Ser. No. 10/985,126, filed Nov.10, 2004; Ser. No. 11/318,822, filed Dec. 27, 2005; Ser. No. 11/318,164,filed Dec. 23, 2005; Ser. No. 11/166,982, filed Jun. 24, 2005; Ser. No.11/140,312, filed May 27, 2005; and U.S. Pat. No. 7,176,043.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. A microelectronic unit comprising: (a) a substrate; and (b) aplurality of microcontacts projecting in a vertical direction from thesubstrate, each microcontact including a base region adjacent thesubstrate and a tip region, remote from the substrate, each microcontacthaving a horizontal dimension which is a first function of verticallocation in the base region and which is a second function of verticallocation in the tip region.
 2. The microelectronic unit of claim 1,wherein the first and second functions are substantially different. 3.The microelectronic unit of claim 1, wherein a slope of horizontaldimension versus vertical location changes abruptly at a boundarybetween the base and the tip regions.
 4. The microelectronic unit ofclaim 1, wherein each of the plurality of microcontacts has alongitudinal axis and a pitch defined by a distance between a first anda second longitudinal axis, wherein the pitch is less than about 200microns.
 5. The microelectronic unit of claim 4, wherein the pitch isless than about 150 microns.
 6. The microelectronic unit of claim 1,wherein there is another region disposed between the base and tipregions.
 7. The microelectronic unit of claim 1, wherein, within each ofthe microcontacts, the base region and the tip region are formed as aunitary body of metal.
 8. The microelectronic unit of claim 1, whereinthe substrate includes a dielectric layer and traces extending along thedielectric layer, at least some of the traces being connected to atleast some of the microcontacts.
 9. A microelectronic unit comprising: asubstrate; a plurality of microcontacts projecting in a verticaldirection from the substrate wherein a pitch between two adjacentmicrocontacts is less than 150 microns.
 10. The microelectronic unit asclaimed in claim 9 wherein the pitch is less than h+d, where h is thevertical height of each microcontact and d is the diameter of eachmicrocontact at a tip of the microcontact, remote from the substrate.11. The microelectronic unit as claimed in claim 9 wherein eachmicrocontact has a height of at least about 50 microns and a tipdiameter of at least about 20 microns.
 12. The microelectronic unit asclaimed in claim 9 wherein each microcontact has a tip with asubstantially flat, horizontal surface.
 13. A microelectronic unitcomprising: (a) a substrate; and (b) a plurality of elongatedmicrocontacts projecting in a vertical direction from the substrate,each microcontact including a base region adjacent the substrate and atip region, remote from the substrate, each microcontact having an axisand a circumferential surface which slopes toward or away from the axisin the vertical direction along the axis, such that the slope of thecircumferential wall changes abruptly at a boundary between the tipregion and the base region.
 14. The microelectronic unit of claim 13,wherein, within each of the microcontacts, the base region and the tipregion are formed as a unitary body of metal.
 15. The microelectronicunit of claim 13, wherein a pitch between adjacent microcontacts is lessthan about 150 microns and each microcontact has a height of about 60 toabout 150 microns.
 16. The microelectronic unit of claim 15, whereineach microcontact has a tip diameter of at least about 20 microns. 17.The microelectronic unit as claimed in claim 13 wherein the pitch isless than h+d, where h is the vertical height of each microcontact and dis the diameter of each microcontact at a tip of the microcontact,remote from the substrate.
 18. The microelectronic unit as claimed inclaim 13 wherein the substrate includes a dielectric layer and tracesextending along the dielectric layer, at least some of the traces beingconnected to at least some of the microcontacts.
 19. The microelectronicunit as claimed in claim 18 wherein the microcontacts project from afirst side of the dielectric layer, the unit also including terminalsexposed at the second side of the dielectric layer and electricallyconnected to at least some of the microcontacts by the traces.
 20. Anassembly including a microelectronic unit as claimed in claim 19 and amicroelectronic element having contacts connected to the microcontacts.21. The assembly of claim 17, wherein there is another region disposedbetween the base and tip regions.
 22. A microelectronic unit comprising:(a) a substrate; and (b) a plurality of microcontacts projecting in avertical direction from the substrate, each microcontact having aproximal portion adjacent the substrate and an elongated distal portionextending from the proximal portion in the vertical direction away fromthe substrate, the width of the post increasing in stepwise fashion atthe juncture between the proximal and distal portions.